Fix typo in Doxygen comments.

Originally committed as revision 8307 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/common.h

@@ -357,7 +357,7 @@ char *av_strdup(const char *s);
 
 /**
  * Frees memory and sets the pointer to NULL.
- * @param arg pointer to the pointer which should be freed
+ * @param ptr pointer to the pointer which should be freed
  */
 void av_freep(void *ptr);